1. Field of the Invention
This invention relates to overvoltage protectors in general and, more particularly, to overvoltage protectors for telephone lines or the like.
2. Description of the Prior Art
Overvoltage protection is necessary to protect both subscriber apparatus and central office equipment from lightning surges and powerline cross. In central offices, there is generally two layers of protection provided: a primary, higher voltage (e.g., 300 volts) and higher power, protector and then a lower voltage (e.g., 90 volts), lower power protector. The first protector usually takes the brunt of any lightning surges and high voltage power line crosses. The lower voltage protector handles the overvoltages that the higher voltage protector either ignores (because the voltage is too low) or manages to get past the primary protector. The voltage limit of the low voltage protector is usually set to a value that would normally cause damage to the central office equipment if left unchecked, such as a power line cross of 120 VAC.
Integrated overvoltage circuits have been available for some time but are not as fast acting as desired. Moreover, the voltage limit of the protector is substantially fixed at the time of manufacture, multiplying the number of different types of protectors needed. For example, the protector for a PBX may have a different voltage limit than that in a central office.
Thus, it is desirable to provide an overvoltage protector having a fast reaction time. Further, it is desirable to provide a protector having a voltage limit that may be varied by the user. Still further, the overvoltage protector should be able to be formed in an integrated circuit, with other circuits if desired, to provide low cost and high reliability.